There is a technology for calculating the state of charge (SOC) of a storage battery based on the correlation between an open circuit voltage (OCV) across the storage battery and the SOC of the storage battery.
Unfortunately, during charging or discharging of the storage battery, it is difficult to directly measure the OCV because a load is connected to the storage battery.
Another approach uses a close circuit voltage (CCV) across a storage battery based on an equivalent circuit, i.e. circuit model; the equivalent circuit equivalently simulates the electrical consequences of the chemical reactions inside the storage battery when a load is connected to the storage battery.
The equivalent circuit of the storage battery is comprised of an unknown OCV across the storage battery, and an internal impedance of the storage battery. The internal impedance of the storage battery is comprised of an internal resistance of the storage battery, and a parallel circuit of a polarization resistance and a capacitance, i.e. an electric double layer capacitance, of the storage battery, which are connected in parallel to each other. The OCV, the internal resistance, and the parallel circuit are connected in series. That is, the voltage across the equivalent circuit of the storage battery serves as the CCV across the storage battery.
Thus, calculating the, value of the CCV based on a predetermined value of the internal impedance during charging or discharging of the storage battery enables the unknown OCV to be calculated. The calculated OCV enables the SOC of the storage battery to be calculated.